1. Field of the Invention
The present invention relates to wireless communication systems and, more particularly, wireless communication systems using backscatter technology.
2. Description of the Related Art
RF Tag systems are radio communication systems that communicate between a radio transceiver, called an Interrogator, and a number of inexpensive devices denoted as Tags. In RF Tag systems, the Interrogator communicates to the Tags using modulated radio signals which activate any Tag in range or may activate a specific Tag within the range. After activating a Tag, the Interrogator may transmit information to it (this is called the Downlink). The Interrogator transmits a Continuous-Wave (CW) radio signal to the Tag; the Tag then modulates the CW signal using modulated backscattering (MBS) in which the Tag is electrically switched by the modulating signal, from being an absorber of RF radiation to a reflector of RF radiation. This modulated backscatter allows communications from the Tag back to the Interrogator (called the Uplink). The Downlink transmission of messages can include information relating to a desired operation of the RF Tag and, for example, the Interrogator is capable of instructing the RF Tag to turn on and/or off on demand. Modulated Backscatter (MBS) systems can be used to manage inventory or perform other useful monitoring application such as reading the state of a sensor.
The operation of an RF Tag system utilizing MBS is now described. In FIG. 1, there is shown an overall block diagram of an RF Tag system. An Application Processor 101 communicates over Local Area Network (LAN) 102 to a plurality of Interrogators 103-104. (Although commonly a plurality of Interrogators 103-104 connected by a LAN 102 to an Application Processor 101 are used, the inventions described herein are also capable of being configured with only a single Interrogator connected either to a LAN or directly to an Application Processor.) The Interrogators may then each communicate with one or more of the Tags 105a-107. For example and with further reference to FIG. 2, the Interrogator 103 receives an information signal, typically from Applications Processor 101. The Interrogator 103 takes this information signal and Processor 200 formats a Downlink message (Information Signal 200a) to be sent to the Tag. The information signal (200a) may include data information such as information specifying which Tag is to respond (each Tag may have fixed or programmed identification number), instructions for the Tag's processor to execute such as activation and deactivation, and/or any other information to be used and/or stored by the Tag's processor. With joint reference to FIGS. 1 and 2, Radio Signal Source 201 synthesizes a radio signal, Modulator 202 modulates the radio signal using Information Signal 200a, and Transmitter 203 transmits this modulated signal via Antenna 204, illustratively using amplitude modulation, to a Tag. Amplitude modulation is a desirable choice because the Tag can demodulate such a signal with a single, inexpensive nonlinear device (such as a diode). However, many modulation schemes are possible such for example, as Phase Shift Keying (PSK) of the subcarrier (e.g., BPSK, QPSK), more complex modulation schemes (e.g., MSK, GMSK), etc.
In the Tag 105a (see FIG. 3a), the reflecting antenna element 301a (e.g. a loop or patch antenna) receives the modulated signal. This signal is demodulated directly to baseband using the Detector/Modulator 302 which, illustratively, may be a single Schottky diode. The result of the diode detector is essentially a demodulation of the incoming signal directly to baseband. The Information Signal 200a is then amplified by Amplifier 303, and bit synchronization is recovered in Clock Recovery Circuit 304. Clock recovery circuits such as circuits that recover a clock from Manchester encoded data are well known in the art. If large amounts of data are being transferred in frames, then frame synchronization may be implemented, as for example by detecting a predetermined bit pattern that indicates the start of a frame. The bit pattern may be detected by clock recovery circuit 304 or processor 305; bit pattern detection is well known in the art. The resulting information from clock recovery circuit 304 is sent to a Processor 305. Processor 305 is typically an inexpensive 4 or 8 bit microprocessor and its associated memory, and it generates an Information Signal 306 from Tag 105a back to the Interrogator (e.g., 103). Information Signal 306 is sent to Detector/Modulator 302 to modulate the RF signal received by Tag 105a to produce a modulated backscatter (i.e. reflected) signal. A Battery 310 or other power supply provides operating power to the circuitry of Tag 10a. Power may also be received, for example, by using inductive coupling or microwaves.
Returning to FIG. 2, the Interrogator 103 receives the reflected modulated signal through Receive Antenna 206, amplifies the signal in a Low Noise Amplifier 207, and demodulates the signal using homodyne detection in a Mixer 208. In an alternative embodiment, a single antenna may replace Transmit antenna (204) and Receive Antenna (206), in which case an electronic method of canceling the transmitted signal from that received by the receiver chain is required.
Using the same Radio Signal Source 201 as is used in the transmit chain means that the demodulation to baseband is done using homodyne detection; this has advantages in that it greatly reduces phase noise in the receiver circuits. The Mixer 208 then sends the Demodulated Signal 209 (if Mixer 208 is a Quadrature Mixer, it would send both I (in phase) and Q (quadrature) signals) to the Filter/Amplifier 210. The resulting filtered signal--typically an Information Signal 213 is sent to a Processor 200 to determine the content of the message.
Generally, RF Tags have a single reflecting antenna. Since the Tag only reflects RF energy instead of generating it, an RF Tag is less expensive to manufacture and requires little battery power when operating. Consequently, an RF Tag provides a low cost arrangement and method of transmitting sensor measurements to a central processing system or operator for evaluation.
The advantages of using RF Tags to transmit information to an Interrogator are accompanied by a disadvantage: since the RF Tag is only a reflector, the signals returned from it are generally weaker than in systems that generate RF energy at both ends of the communications link. For example, in an RF Tag system the signal-to-noise ratio (SNR) of a signal sent from the Tag falls off rapidly (proportionally to r.sup.-4, where r is the distance between the transmitter and reflector). By comparison, in a communication system having a transmitter at one end and a receiver at the other, the SNR in each direction falls off slowly (proportionally to r.sup.-2). Thus, the interrogation range of the RF Tag is notably more limited by its distance from the RF transmitting source.
FIG. 4 depicts incoming RF radiation generated by RF Interrogator 103 and directed toward an RF Tag 105a having a reflecting antenna element 301a, and FIG. 5 shows the reflectance of the RF Tag 105a of FIG. 4. As is clear from FIG. 5, during operation the reflecting antenna 301a of RF Tag 105a is either in a fully reflecting mode or an essentially non-reflecting mode. For each fill period T of the square wave depicted in FIG. 5, reflecting antenna 301a of RF Tag 105a is only in the fully reflecting mode for half of each period T.